Added Plugin for gzip compression

The plugin checks for compatibilty with `gzcompress`  function (http://php.net/manual/es/function.gzcompress.php)

Allows some configuration:

```php
$options = array(
   'level' => 6; //Default is 9 (maximum)
);
$this->pluginsConfig["AmfphpGzip"] = $options;
```